Main loop: Main flight loop (Part 13 of 16)

       Name: Main flight loop (Part 13 of 16)                        [Show more]
       Type: Subroutine
   Category: Main loop
    Summary: Charge shields and energy banks
  Deep dive: Program flow of the main game loop
             Scheduling tasks with the main loop counter
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


 The main flight loop covers most of the flight-specific aspects of Elite. This
 section covers the following:

   * Charge shields and energy banks (every 7 iterations of the main loop)


.MA18

                        \ --- Mod: Code removed for Elite-A: ------------------>

\ LDA BOMB              \ If we set off our energy bomb (see MA24 above), then
\ BPL MA77              \ BOMB is now negative, so this skips to MA21 if our
\                       \ energy bomb is not going off
\
\ ASL BOMB              \ We set off our energy bomb, so rotate BOMB to the
\                       \ left by one place. BOMB was rotated left once already
\                       \ during this iteration of the main loop, back at MA24,
\                       \ so if this is the first pass it will already be
\                       \ %11111110, and this will shift it to %11111100 - so
\                       \ if we set off an energy bomb, it stays activated
\                       \ (BOMB > 0) for four iterations of the main loop
\
\ JSR WSCAN             \ Call WSCAN to wait for the vertical sync, so the whole
\                       \ screen gets drawn and the following palette change
\                       \ won't kick in while the screen is still refreshing
\
\ LDA #%00110000        \ Set the palette byte at SHEILA &21 to map logical
\ STA VIA+&21           \ colour 0 to physical colour 7 (white), but with only
\                       \ one mapping (rather than the 7 mappings required to
\                       \ do the mapping properly). This makes the space screen
\                       \ flash with black and white stripes. See p.382 of the
\                       \ Advanced User Guide for details of why this single
\                       \ palette change creates a special effect

                        \ --- End of removed code ----------------------------->

.MA77

 LDA MCNT               \ Fetch the main loop counter and calculate MCNT mod 7,
 AND #7                 \ jumping to MA22 if it is non-zero (so the following
 BNE MA22               \ code only runs every 8 iterations of the main loop)

 LDX ENERGY             \ Fetch our ship's energy levels and skip to b if bit 7
 BPL b                  \ is not set, i.e. only charge the shields from the
                        \ energy banks if they are at more than 50% charge

 LDX ASH                \ Call SHD to recharge our aft shield and update the
 JSR SHD                \ shield status in ASH
 STX ASH

 LDX FSH                \ Call SHD to recharge our forward shield and update
 JSR SHD                \ the shield status in FSH
 STX FSH

.b

 SEC                    \ Set A = ENERGY + ENGY + 1, so our ship's energy
 LDA ENGY               \ level goes up by the correct amount for our current
 ADC ENERGY             \ ship, depending on whether we have an energy unit
                        \ fitted

 BCS P%+5               \ If the value of A did not overflow (the maximum
 STA ENERGY             \ energy level is &FF), then store A in ENERGY